Heterogeneously catalyzed partial gas phase oxidation of propene to acrylic acid

ABSTRACT

In a process for heterogeneously catalyzed partial gas phase oxidation of propene to acrylic acid, the starting reaction gas mixture is oxidized at a propene loading of &lt;160 l(STP)/l·h in a first reaction stage over a fixed catalyst bed  1  which is accommodated in two successive reaction zones A, B, and the acrolein-containing product gas mixture of the first reaction stage is subsequently oxidized in a second reaction stage over a fixed catalyst bed  2  which is a accommodated in two successive reaction zones C, D, the highest temperature of the reaction gas mixture within reaction zone A being above the highest temperature of the reaction gas mixture within reaction zone B and the highest temperature of the reaction gas mixture within reaction zone C being above the highest temperature of the reaction gas mixture within reaction zone D.

[0001] The present invention relates to a process for partiallyoxidizing propene to acrylic acid in the gas phase under heterogeneouscatalysis by conducting a starting reaction gas mixture 1 whichcomprises propene, molecular oxygen and at least one inert gas andcontains the molecular oxygen and the propene in a molar O₂: C₃H₆ ratioof ≧1 in a first reaction stage over a fixed catalyst bed 1 which isarranged in two spatially successive reaction zones A, B, thetemperature of reaction zone A being a temperature in the range from 290to 380° C. and the temperature of reaction zone B likewise being atemperature in the range from 290 to 380° C., and whose activecomposition is at least one multimetal oxide comprising the elements Mo,Fe and Bi, in such a way that reaction zone A extends to a propeneconversion of from 40 to 80 mol % and the propene conversion on singlepass through the fixed catalyst bed 1 is ≧90 mol % and the accompanyingselectivity of acrolein formation and also of acrylic acid by-productiontaken together is ≧90 mol %, the temperature of the product gas mixtureleaving the first reaction stage is optionally reduced by cooling andmolecular oxygen and/or inert gas are optionally added to the productgas mixture, and then the product gas mixture, as a starting reactiongas mixture 2 comprising acrolein, molecular oxygen and at least oneinert gas and containing the molecular oxygen and the acrolein in amolar O₂: C₃H₄O ratio of ≧0.5, is conducted in a second reaction stageover a fixed catalyst bed 2 which is arranged in two spatiallysuccessive reaction zones C, D, the temperature of reaction zone C beinga temperature in the range from 230 to 320° C. and the temperature ofreaction zone D likewise being a temperature in the range from 230 to320° C., and whose active composition is at least one multimetal oxidecomprising the elements Mo and V, in such a way that reaction zone Cextends to an acrolein conversion of from 45 to 85 mol % and theacrolein conversion on single pass through the fixed catalyst bed 2 is≧90 mol % and the selectivity of acrylic acid formation assessed overall reaction zones, based on converted propene, is ≧80 mol %, thesequence in time in which the reaction gas mixture flows through thereaction zones corresponding to the alphabetic sequence of the reactionzones.

[0002] Acrylic acid is an important monomer which finds use as such orin the form of its acrylic esters for obtaining polymers suitable, forexample, as adhesives.

[0003] On the industrial scale, acrylic acid is prepared predominantlyby heterogeneously catalyzed partial gas phase oxidation of propene(cf., for example, WO 01/36364).

[0004] In principle, the heterogeneously catalyzed partial gas phaseoxidation of propene to acrylic acid proceeds in two chronologicallysuccessive steps.

[0005] In the first step, propene is oxidized to acrolein, and in thesecond step, the acrolein formed in the first step is oxidized toacrylic acid.

[0006] Typically, the heterogeneously catalyzed partial gas phaseoxidation of propene to acrylic acid is therefore carried out atelevated temperatures in two spatially successive reaction stages, eachof the two reaction stages using catalytically active multimetal oxidecompositions which are tailored to the oxidation step to be catalyzed inthe particular reaction stage. The multimetal oxide active compositionsto be used in the first reaction stage for the step from propene toacrolein comprise the elements Mo, Fe and Bi, while the multimetal oxideactive compositions to be used in the second reaction stage for the stepfrom acrolein to acrylic acid comprise the elements Mo and V. Nointermediate separation of the acrolein from the product gas mixture ofthe first reaction stage is normally carried out. Rather, theacrolein-containing product gas mixture of the first reaction stage,optionally after reducing its temperature by cooling, and alsooptionally after adding further molecular oxygen and/or inert gas, isgenerally used as such for the further oxidation of acrolein in thesecond reaction stage.

[0007] In addition to molecular oxygen and the reactants, the reactiongas mixture comprises inert gas in order to keep the reaction gasmixture outside the explosion range, among other reasons.

[0008] One objective of such a two-stage heterogeneously catalyzedpartial gas phase oxidation of propene to acrylic acid is to achieve avery high yield Y^(AA) of acrylic acid (this is the number of moles ofpropene converted to acrylic acid, based on the number of moles ofpropene used) on single pass of the reaction gas mixture through the tworeaction stages under otherwise predefined boundary conditions.

[0009] A further objective of such a heterogeneously catalyzed partialgas phase oxidation of propene to acrylic acid is to achieve a very highspace-time yield STY^(AA) of acrylic acid (in a continuous procedure,this is the total amount of acrylic acid obtained per hour and unit oftotal volume of the fixed catalyst bed used in liters).

[0010] At a constant given yield Y^(AA), the greater the hourly spacevelocity of propene on the fixed catalyst bed of the first reactionstage (fixed catalyst bed 1) (this refers to the amount of propene inliters at STP (=l (STP); the volume in liters which would be taken up bythe appropriate amount of propene under standard conditions, i.e. at 25°C. and 1 bar) which is conducted as a constituent of the startingreaction gas mixture 1 through one liter of fixed catalyst bed 1 perhour), the higher the space-time yield.

[0011] The teachings of the documents WO 01/36364, DE-A 19927624, DE-A19948248, DE-A 19948523, DE-A 19948241 and DE-A 19910506 are thereforedirected toward significantly increasing the hourly space velocity ofpropene on the fixed catalyst bed of the first reaction stage atsubstantially constant Y^(AA) This is achieved substantially byarranging both the fixed catalyst bed in the first reaction stage andthe fixed catalyst bed in the second reaction stage each in twospatially successive temperature zones (reaction zones). The hourlyspace velocity of propene on the fixed catalyst bed 1 is then selectedat ≧160 l (STP)/l of fixed catalyst bed 1·h and the temperature of thesecond (in the flow direction of the reaction gas mixture) temperaturezone in each case has to be at least 5° C. above the temperature of thefirst temperature zone.

[0012] In a similar manner, EP-A 1106598 also teaches a process of thehigh load method for the heterogeneously catalyzed partial gas phaseoxidation of propene to acrylic acid, in which the fixed catalyst bedsof the two reaction stages are each arranged in a plurality oftemperature zones. According to the teaching of EP-A 1106598, thetemperature difference of a subsequent temperature zone in the flowdirection of the reaction gas mixture can be either more or less than 5°C. above the temperature of the preceding temperature zone, and EP-A1106598 leaves completely open the question of under which conditions alarger and under which conditions a smaller temperature differenceshould be applied.

[0013] EP-A 1106598 also leaves completely open the question of what thetemperature of a reaction zone or a temperature zone means.

[0014] In contrast, in the remaining prior art documents cited, thetemperature of a reaction zone refers to the temperature of the fixedcatalyst bed disposed in the reaction zone when performing the processin the absence of a chemical reaction. When this temperature within thereaction zone is not constant, the term temperature of a reaction zonerefers to the (numerical) mean of the temperature of the fixed catalystbed along the reaction zone. It is essential that the individualreaction zones are heated substantially independently of one another, sothat a reaction zone always corresponds to a temperature zone. Theaforementioned definition of the temperature of a reaction zone alsoapplies in this document.

[0015] Since the heterogeneously catalyzed partial gas phase oxidationof propene to acrylic acid is a markedly exothermic reaction, thetemperature of the reaction gas mixture on reactive pass through thefixed catalyst bed is generally different from the temperature of areaction zone. It is normally above the temperature of the reaction zoneand, within a reaction zone, generally passes through a maximum (heatingpoint maximum) or falls starting from a maximum value.

[0016] However, a disadvantage of the teachings of the prior art is thatthey are exclusively directed toward operating a multizone arrangementunder a high propene loading. This is disadvantageous in that such aprocedure is inevitably accompanied by a high STY^(AA). However, thisassumes a corresponding market requirement for acrylic acid. When thelatter is absent (for example temporarily), the multizone arrangementthen necessarily has to be operated at lower propene loadings, andanother target parameter which then comes to the forefront is a veryhigh selectivity of acrylic acid formation, based on converted propene(S^(AA)). This is the molar amount of acrylic acid formed on single passthrough the multizone arrangement, based on the number of moles ofconverted propene.

[0017] It is an object of the present invention to provide a process forheterogeneously catalyzed partial gas phase oxidation of propene toacrylic acid in a multizone arrangement, in which acrylic acid is formedwith very high selectivity at propene loadings of <160 l (STP)/l·h.

[0018] We have found that this object is achieved by a process forpartially oxidizing propene to acrylic acid in the gas phase underheterogeneous catalysis by conducting a starting reaction gas mixture 1which comprises propene, molecular oxygen and at least one inert gas andcontains the molecular oxygen and the propene in a molar O₂: C₃H₆ ratioof ≧1 in a first reaction stage over a fixed catalyst bed 1 which isarranged in two spatially successive reaction zones A, B, thetemperature of reaction zone A being a temperature in the range from 290to 380° C. and the temperature of reaction zone B likewise being atemperature in the range from 290 to 380° C., and whose activecomposition is at least one multimetal oxide comprising the elements Mo,Fe and Bi, in such a way that reaction zone A extends to a propeneconversion of from 40 to 80 mol % and the propene conversion on singlepass through the fixed catalyst bed 1 is ≧90 mol % and the accompanyingselectivity of acrolein formation and also of acrylic acid by-productiontaken together is ≧90 mol %, the temperature of the product gas mixtureleaving the first reaction stage is optionally reduced by cooling andmolecular oxygen and/or inert gas are optionally added to the productgas mixture, and then the product gas mixture, as a starting reactiongas mixture 2 comprising acrolein, molecular oxygen and at least oneinert gas and containing the molecular oxygen and the acrolein in amolar O₂: C₃H₄O ratio of ≧0.5, is conducted in a second reaction stageover a fixed catalyst bed 2 which is arranged in two spatiallysuccessive reaction zones C, D, the temperature of reaction zone C beinga temperature in the range from 230 to 320° C. and the temperature ofreaction zone D likewise being a temperature in the range from 230 to320° C., and whose active composition is at least one multimetal oxidecomprising the elements Mo and V, in such a way that reaction zone Cextends to an acrolein conversion of from 45 to 85 mol % and theacrolein conversion on single pass through the fixed catalyst bed 2 is≧90 mol % and the selectivity of acrylic acid formation assessed overall reaction zones, based on converted propene, is ≧80 mol %, thesequence in time in which the reaction gas mixture flows through thereaction zones corresponding to the alphabetic sequence of the reactionzones, wherein

[0019] a) the hourly space velocity of the propene contained in thestarting reaction gas mixture 1 on the fixed catalyst bed 1 is <160 l(STP) of propene/l of fixed catalyst bed 1·h and ≧90 l (STP) ofpropene/l of fixed catalyst bed 1·h;

[0020] b) the volume-specific activity of the fixed catalyst bed 1 iseither constant or increases at least once in the flow direction of thereaction gas mixture over the fixed catalyst bed 1;

[0021] c) the difference T^(maxA)−T^(maxB), formed from the highesttemperature T^(maxA) which the reaction gas mixture has within reactionzone A and the highest temperature T^(maxB) which the reaction gasmixture has within reaction zone B, is ≧0° C.;

[0022] d) the hourly space velocity of the acrolein contained in thestarting reaction gas mixture 2 on the fixed catalyst bed 2 is ≦145 l(STP) of acrolein/l of fixed catalyst bed 2·h and ≧70 l (STP) ofacrolein/l of fixed catalyst bed 2·h;

[0023] e) the volume-specific activity of the fixed catalyst bed 2increases at least once in the flow direction of the reaction gasmixture over the fixed catalyst bed 2; and

[0024] f) the difference T^(maxC)−T^(maxD), formed from the highesttemperature T^(maxC) which the reaction gas mixture has within reactionzone C and the highest temperature T^(maxD) which the reaction gasmixture has within reaction zone D, is ≧0° C.

[0025] In general, the difference T^(maxA)−T^(maxB) in the processaccording to the invention will not be more than 80° C.

[0026] According to the invention, preference is given toT^(maxA)−T^(maxB) being ≧3° C. and ≦70° C. Very particular preference isgiven to T^(maxA)−T^(maxB) in the process according to the inventionbeing ≧20° C. and ≦60° C.

[0027] Also, the difference T^(maxC)−T^(maxD) in the process accordingto the invention will generally not be more than 75° C.

[0028] According to the invention, preference is given toT^(maxC)−T^(maxD) being ≧3° C. and ≦60° C. Very particular preference isgiven to T^(maxC)−T^(maxD) in the process according to the inventionbeing ≧5° C. and ≦40° C.

[0029] The process according to the invention proves advantageous, forexample, when the hourly space velocity of the propene contained in thestarting reaction gas mixture 1 on the fixed catalyst bed 1 is ≧90 l(STP) of propene/l·h and ≦155 l (STP) of propene/l·h, or ≧100 l (STP)propene/l·h and ≦150 l (STP) pf propene/l·h, or ≧110 l (STP) ofpropene/l·h and ≦145 l (STP) of propene/l·h, or ≧120 l (STP) ofpropene/l·h and ≦140 l (STP) of propene/l·h, or ≧125 l (STP) ofpropene/l·h and ≦135 l (STP) of propene/l·h, and the hourly spacevelocity of the acrolein contained in the starting reaction gas mixture2 on the fixed catalyst bed 2 is ≧70 l (STP) of acrolein/l·h and ≦140 l(STP) of acrolein/l·h, or ≦135 l (STP) of acrolein/l·h, or ≧80 l (STP)of acrolein/l·h and ≦130 l (STP) of acrolein/l·h, or ≧90 l (STP) ofacrolein/l·h and ≦125 l (STP) of acrolein/l·h, or ≧100 l (STP) ofacrolein/l o h and ≦1 20 l (STP) of acrolein/l·h, or ≧105 l (STP) ofacrolein/l·h and ≦115 l (STP) of acrolein/l·h.

[0030] It will be appreciated that the process according to theinvention can also be applied when the hourly space velocity of thepropene contained in the starting reaction gas mixture 1 on the fixedcatalyst bed 1 is <90 l (STP) of propene/l·h and the hourly spacevelocity of the acrolein contained in the starting reaction gas mixture2 on the fixed catalyst bed 2 is <70 l (STP) of acrolein/l·h. However,the operation of a multi-zone arrangement at such low reactant loadingswould hardly be economically viable.

[0031] When performing the process according to the invention, thedifferences T^(maxA)−T^(maxB) required in accordance with the inventionare normally attained when, on the one hand, both the temperature ofreaction zone A and the temperature of reaction zone B are in the rangefrom 290 to 380° C. and, on the other hand, the difference between thetemperature of reaction zone B (T_(B)) and the temperature of reactionzone A (T_(A)), i.e. T_(B)−T_(A), is ≦0°C. and ≧−20° C. or ≧−10° C., or≦0° C. and ≧−5° C., or frequently ≦0° C. and ≧−3° C.

[0032] In other words, in contrast to the teaching of the prior art forhigh loadings, the temperature of the subsequent zone in the processaccording to the invention will normally be lower than the temperaturein the preceding reaction zone.

[0033] This also applies to the two reaction zones of the secondreaction stage.

[0034] In other words, when performing the process according to theinvention, the differences T^(maxC)−T^(maxD) required in accordance withthe invention are normally attained when, on the one hand, both thetemperature of reaction zone C and the temperature of reaction zone Dare in the range from 230 to 320° C. and, on the other hand, thedifference between the temperature of reaction zone D (T_(D)) and thetemperature of reaction zone C (T_(C)), i.e. T_(D)−T_(C), is ≦0° C. and≧−20° C. or ≧−10° C., or ≦0° C. and ≧−5° C., or frequently ≦0° C. and≧−3° C.

[0035] The aforementioned statement relating to the temperaturedifference T_(B)−T_(A) also applies when the temperature of reactionzone A is within the preferred temperature range of from 305 to 365° C.,or within the particularly preferred range of from 310 to 340° C.

[0036] In a corresponding manner, the aforementioned statement relatingto the temperature difference T_(D)−T_(C) also applies when thetemperature of reaction zone C is within the preferred temperature rangeof from 250 to 300° C., or within the particularly preferred range offrom 260° C. to 280° C.

[0037] The working pressure in both reaction stages of the processaccording to the invention can either be below atmospheric pressure (forexample down to 0.5 bar) or above atmospheric pressure. Typically, theworking pressure in the two reaction stages of the process according tothe invention will be at values of from 1 to 5 bar, frequently from 1 to3 bar. Normally, the reaction pressure will not exceed 100 bar in eitherof the two reaction stages.

[0038] In general, the propene conversion based on single pass of thefixed catalyst bed 1 in the process according to the invention will be≧92 mol % or ≧94 mol %.

[0039] The selectivity of product of value formation (sum of acroleinformation and acrylic acid by-production) in the case of suitablecatalyst selection in a manner known per se will regularly be ≧92 mol %,or ≧94 mol %, frequently ≧95 mol %, or ≧96 mol % or ≧97 mol %.

[0040] According to the invention, preference is given to reaction zoneA extending to a propene conversion of from 50 to 70 mol % and morepreferably to a propene conversion of from 60 to 70 mol %.

[0041] In general, the hourly space velocity of acrolein on the fixedcatalyst bed 2 in the process according to the invention will be about10 l (STP)/l·h, frequently about 20 to 25 l (STP)/l·h, below the hourlyspace velocity of propene on the fixed catalyst bed 1. This can beprimarily attributed to neither the conversion of propene nor theselectivity of acrolein formation generally attaining 100%.

[0042] According to the invention, preference is given to reaction zoneC extending to a conversion of the acrolein coming from the firstreaction stage of from 50 to 85 mol % or from 60 to 85 mol %.

[0043] In general, the acrolein conversion based on single pass of thefixed catalyst bed 2 in the process according to the invention will be≧92 mol % or ≧94 mol % or ≧96 mol % or ≧98 mol % and frequently even ≧99mol % or more.

[0044] When the fixed catalyst bed 2 is selected appropriately in amanner known per se, the selectivity of acrylic acid formation assessedover both reaction stages in the process according to the invention,based on converted propene, may be at values of ≧83 mol %, frequently at≧85 mol %, or ≧88 mol %, often at ≧90 mol %, or ≧93 mol %.

[0045] According to the invention, the molar O₂: C₃H₆ ratio in thestarting reaction gas mixture 1 has to be ≧1. Typically, this ratio willbe at values of ≦3. According to the invention, the molar O₂: C₃H₆ ratioin the starting reaction gas mixture 1 will frequently be ≧1.5 and ≦2.0.

[0046] According to the invention, the molar O₂: C₃H₄O ratio in thestarting reaction gas mixture 2 is preferably likewise ≧1. Typically,this ratio will likewise be at values of ≦3. According to the invention,the molar O₂: acrolein ratio in the starting reaction gas mixture 2 willfrequently be from 1 to 2 or from 1 to 1.5.

[0047] Useful catalysts for the fixed catalyst bed 1 of the processaccording to the invention include all of those catalysts whose activecomposition is at least one multimetal oxide comprising Mo, Bi and Fe.

[0048] These are in particular the multimetal oxide active compositionsof the general formula I of DE-A 19955176, the multimetal oxide activecompositions of the general formula I of DE-A 19948523, the multimetaloxide active compositions of the general formulae I, II and III of DE-A10101695, the multimetal oxide active compositions of the generalformulae I, II and III of DE-A 19948248 and the multimetal oxide activecompositions of the general formulae I, II and III of DE-A 19955168 andalso the multimetal oxide active compositions specified in EP-A 700714.

[0049] Also suitable for the fixed catalyst bed 1 are the multimetaloxide catalysts comprising Mo, Bi and Fe which are disclosed in thedocuments DE-A 10046957, DE-A 10063162, DE-C 3338380, DE-A 19902562,EP-A 15565, DE-C 2380765, EP-A 807465, EP-A 279374, DE-A 3300044, EP-A575897, US-A 4438217, DE-A 19855913, WO 98/24746, DE-A 19746210 (thoseof the general formula II), JP-A 91/294239, EP-A 293224 and EP-A 700714.This applies in particular for the exemplary embodiments in thesedocuments, and among these particular preference is given to those ofEP-A 15565, EP-A 575897, DE-A 19746210 and DE-A 19855913. Particularemphasis is given in this context to a catalyst according to example 1cfrom EP-A 15565 and also to a catalyst to be prepared in a correspondingmanner but having the active compositionMo₁₂Ni₆₅Zn₂Fe₂Bi₁P_(0.0065)K_(0.06)Ox.10 SiO₂. Emphasis is also given tothe example having the serial number 3 from DE-A 19855913(stoichiometry: Mo₁₂Co₇Fe₃Bi_(0.6)K_(0.08)Si_(1.6)Ox) as an unsupportedhollow cylinder catalyst of geometry 5 mm×3 mm×2 mm or 5 mm×2 mm×2 mm(each external diameter×height×internal diameter) and also to theunsupported multimetal oxide 11 catalyst according to example 1 of DE-A19746210. Mention should also be made of the multimetal oxide catalystsof U.S. Pat. No. 4438217. The latter is true in particular when thesehave a hollow cylinder geometry of the dimensions 5.5 mm×3 mm×3.5 mm, or5 mm×2 mm×2 mm, or 5 mm×3 mm×2 mm, or 6 mm×3 mm×3 mm, or 7 mm×3 mm×4 mm(each external diameter×height×internal diameter). Likewise suitable arethe multimetal oxide catalysts and geometries of DE-A 10101695 or WO02/062737.

[0050] Also suitable are example 1 of DE-A 10046957 (stoichiometry:[Bi₂W₂O₉×2WO₃]_(0.5). [Mo₁₂Co_(5.6)Fe_(2.94)Si_(1.59)K_(0.08)O_(x)]₁) asan unsupported hollow cylinder (ring) catalyst of geometry 5 mm×3 mm×2mm or 5 mm×2 mm×2 mm (each external diameter×length×internal diameter),and also the coated catalysts 1, 2 and 3 of DE-A 10063162(stoichiometry: Mo₁₂Bi_(1.0)Fe₃Co₇Si_(1.6)K_(0.08)), except as annularcoated catalysts of appropriate coating thickness and applied to supportrings of geometry 5 mm×3 mm×1.5 mm or 7 mm×3 mm×1.5 mm (each externaldiameter×length×internal diameter).

[0051] A multiplicity of the multimetal oxide active compositionssuitable for the catalysts of the fixed catalyst bed 1 can beencompassed by the general formula I

Mo₁₂Bi_(a)Fe_(b)X¹ _(c)X² _(d)X³ _(e)X⁴ _(f)On  (I)

[0052] where the variables are defined as follows:

[0053] X¹=nickel and/or cobalt,

[0054] X²=thallium, an alkali metal and/or an alkaline earth metal,

[0055] X³=zinc, phosphorus, arsenic, boron, antimony, tin, cerium, leadand/or tungsten,

[0056] X⁴=silicon, aluminum, titanium and/or zirconium,

[0057] a=from 0.5 to 5,

[0058] b=from 0.01 to 5, preferably from 2 to 4,

[0059] c=from 0 to 10, preferably from 3 to 10,

[0060] d=from 0 to 2, preferably from 0.02 to 2,

[0061] e=from 0 to 8, preferably from 0 to 5,

[0062] f=from 0 to 10 and

[0063] n=a number which is determined by the valency and frequency ofthe elements other than oxygen in 1.

[0064] They are obtainable in a manner known per se (see, for example,DE-A 4023239) and are customarily shaped undiluted to give spheres,rings or cylinders or else used in the form of coated catalysts, i.e.preshaped inert support bodies coated with the active composition. Itwill be appreciated that they may also be used as catalysts in powderform.

[0065] In principle, active compositions of the general formula I can beprepared in a simple manner by obtaining a very intimate, preferablyfinely divided dry mixture corresponding to their stoichiometry fromsuitable sources of their elemental constituents and calcining it attemperatures of from 350 to 650° C. The calcination may be effectedeither under inert gas or under an oxygen atmosphere, for example air(mixture of inert gas and oxygen) and also under a reducing atmosphere(for example mixture of inert gas, NH₃, CO and/or H₂). The calcinationtime can be from a few minutes to a few hours and typically decreaseswith temperature. Useful sources for the elemental constituents of themultimetal oxide active compositions I are those compounds which arealready oxides and/or those compounds which can be converted to oxidesby heating, at least in the presence of oxygen.

[0066] In addition to the oxides, such useful starting compounds includein particular halides, nitrates, formates, oxalates, citrates, acetates,carbonates, amine complexes, ammonium salts and/or hydroxides (compoundssuch as NH₄OH, (NH₄)₂CO₃, NH₄NO₃, NH₄CHO₂, CH₃COOH, NH₄CH₃CO₂ and/orammonium oxalate which decompose and/or can be decomposed on latercalcining at the latest to release compounds in gaseous form can beadditionally incorporated into the dry mixture).

[0067] The starting compounds for preparing the multimetal oxide activecompositions I can be intimately mixed in dry or in wet form. When theyare mixed in dry form, the starting compounds are advantageously used asfinely divided powders and subjected to calcination after mixing andoptional compacting. However, preference is given to intimate mixing inwet form. Customarily, the starting compounds are mixed with each otherin the form of an aqueous solution and/or suspension. Particularlyintimate dry mixtures are obtained in the mixing process described whenthe starting materials are exclusively sources of the elementalconstituents in dissolved form. The solvent used is preferably water.Subsequently, the aqueous composition obtained is dried, and the dryingprocess is preferably effected by spray-drying the aqueous mixture atexit temperatures of from 100 to 150° C.

[0068] Typically, the multimetal oxide active compositions of thegeneral formula I are used in the fixed catalyst bed 1 not in powderform, but rather shaped into certain catalyst geometries, and theshaping may be effected either before or after the final calcination.For example, unsupported catalysts can be prepared from the powder formof the active composition or its uncalcined and/or partially calcinedprecursor composition by compacting to the desired catalyst geometry(for example by tableting or extruding), optionally with the addition ofassistants, for example graphite or stearic acid as lubricants and/orshaping assistants and reinforcing agents such as microfibers of glass,asbestos, silicon carbide or potassium titanate. Examples of unsupportedcatalyst geometries include solid cylinders or hollow cylinders havingan external diameter and a length of from 2 to 10 mm. In the case of thehollow cylinder, a wall thickness of from 1 to 3 mm is advantageous. Itwill be appreciated that the unsupported catalyst can also havespherical geometry, and the spherical diameter can be from 2 to 10 mm.

[0069] A particularly advantageous hollow cylinder geometry is 5 mm×3mm×2 mm (external diameter×length×internal diameter), in particular inthe case of unsupported catalysts.

[0070] It will be appreciated that the pulverulent active composition orits pulverulent precursor composition which is yet to be calcined and/orpartially calcined can be shaped by applying to preshaped inert catalystsupports. The coating of the support bodies to produce the coatedcatalysts is generally performed in a suitable rotatable vessel, asdisclosed, for example, by DE-A 2909671, EP-A 293859 or EP-A 714700. Tocoat the support bodies, the powder composition to be applied isadvantageously moistened and dried again after application, for exampleby means of hot air. The coating thickness of the powder compositionapplied to the support body is advantageously selected within the rangefrom 10 to 1000 μm, preferably within the range from 50 to 500 μm andmore preferably within the range from 150 to 250 μm. Useful supportmaterials are the customary porous or nonporous aluminum oxides, silicondioxide, thorium dioxide, zirconium dioxide, silicon carbide orsilicates such as magnesium silicate or aluminum silicate. Theygenerally behave inertly with regard to the target reaction on which theprocess according to the invention in the first reaction stage is based.The support bodies can have a regular or irregular shape, althoughpreference is given to regularly shaped support bodies having distinctsurface roughness, for example spheres or hollow cylinders. It issuitable to use substantially nonporous, surface-roughened sphericalsupports made of steatite (e.g. Steatite C220 from CeramTec) whosediameter is from 1 to 8 mm, preferably from 4 to 5 mm. However, suitablesupport bodies also include cylinders whose length is from 2 to 10 mmand whose external diameter is from 4 to 10 mm. In the case of ringssuitable as support bodies according to the invention, the wallthickness is also typically from 1 to 4 mm. According to the invention,annular support bodies to be used preferably have a length of from 2 to6 mm, an external diameter of from 4 to 8 mm and a wall thickness offrom 1 to 2 mm. Suitable as support bodies according to the inventionare in particular rings of the geometry 7 mm×3 mm×4 mm (externaldiameter×length×internal diameter). It will be appreciated that thefineness of the catalytically active oxide compositions to be applied tothe surface of the support body will be adapted to the desired coatingthickness (cf. EP-A 714 700).

[0071] Suitable multimetal oxide active compositions for the catalystsof the first reaction stage are also compositions of the general formulaII

[Y¹ _(a′)Y² _(b′)O_(x′)]_(p)[Y³ _(c′)Y⁴ _(d′)Y⁵ _(e′)Y⁶ _(f′)Y⁷ _(g′)Y²_(h′)O_(y′)]_(q)  (II)

[0072] where the variables are defined as follows:

[0073] Y¹=only bismuth or bismuth and at least one of the elementstellurium, antimony, tin and copper,

[0074] Y²=molybdenum or molybdenum and tungsten,

[0075] Y³=an alkali metal, thallium and/or samarium,

[0076] Y⁴=an alkaline earth metal, nickel, cobalt, copper, manganese,zinc, tin, cadmium and/or mercury,

[0077] Y⁵=iron or iron and at least one of the elements chromium andcerium,

[0078] Y⁶=phosphorus, arsenic, boron and/or antimony,

[0079] Y⁷=a rare earth metal, titanium, zirconium, niobium, tantalum,rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium,silicon, germanium, lead, thorium and/or uranium,

[0080] a′=from 0.1 to 8,

[0081] b′=from 0.1 to 30,

[0082] c′=from 0 to 4,

[0083] d′=from 0 to 20,

[0084] e′=≧0 to 20,

[0085] f′=from 0 to 6,

[0086] g′=from 0 to 15,

[0087] h′=from 8 to 16,

[0088] x′,y′=numbers which are determined by the valency and frequencyof the elements other than oxygen in II and

[0089] p,q=numbers whose p/q ratio is from 0.1 to 10,

[0090] comprising three-dimensional regions of the chemical compositionY¹ _(a)Y² _(b′O) ^(x′) which are delimited from their local environmentas a consequence of their different chemical composition from theirlocal environment, and whose maximum diameter (longest line through thecenter of the region and connecting two points on the surface(interface)) is from 1 nm to 100 μm, frequently from 10 nm to 500 nm orfrom 1 μm to 50 or 25 μm.

[0091] Particularly advantageous multimetal oxide compositions IIaccording to the invention are those in which Y¹ is only bismuth.

[0092] Among these, preference is given in turn to those of the generalformula III

[Bi_(a″)Z² _(b″)O_(x″)]_(p″) [Z² ₁₂Z³ _(c″)Z⁴ _(d″)Fe_(e″)Z⁵ _(f″)Z⁶_(g″)Z⁷ _(h″)O_(y″)]_(q″)  (III)

[0093] where the variables are defined as follows:

[0094] Z²=molybdenum or molybdenum and tungsten,

[0095] Z³=nickel and/or cobalt,

[0096] Z⁴=thallium, an alkali metal and/or an alkaline earth metal,

[0097] Z⁵=phosphorus, arsenic, boron, antimony, tin, cerium and/or lead,

[0098] Z⁶=silicon, aluminum, titanium and/or zirconium,

[0099] Z⁷=copper, silver and/or gold,

[0100] a″=from 0.1 to 1,

[0101] b″=from 0.2 to 2,

[0102] c″=from 3 to 10,

[0103] d″=from 0.02 to 2,

[0104] e″=from 0.01 to 5, preferably from 0.1 to 3,

[0105] f″=from 0 to 5,

[0106] g″=from 0 to 10,

[0107] h″from 0 to 1,

[0108] x″,y″=numbers which are determined by the valency and frequencyof the elements other than oxygen in III, p″,q″=numbers whose p″/q″ratio is from 0.1 to 5, preferably from 0.5 to 2, and particularpreference is given to those compositions III in which Z²_(b″)=(tungsten)_(b′) and Z² ₁₂=(molybdenum)₁₂.

[0109] It is also advantageous when at least 25 mol % (preferably atleast 50 mol % and more preferably at least 100 mol %) of the totalproportion of [Y¹ _(a′)Y² _(b′)O_(x′)]_(p) ([Bi_(a″)Z² _(b′)O_(x″])_(p″)) of the multimetal oxide compositions II (multimetal oxidecompositions II) suitable according to the invention in the multimetaloxide compositions II (multimetal oxide compositions III) suitableaccording to the invention are in the form of three-dimensional regionsof the chemical composition Y¹ _(a′)Y² _(b′)O_(x′)[Bi_(a″)Z²_(b″)O_(x″)] which are delimited from their local environment as aconsequence of their different chemical composition from their localenvironment, and whose maximum diameter is in the range from 1 nm to 100μm.

[0110] With regard to the shaping, the statements made for themultimetal oxide I catalysts apply to the multimetal oxide II catalysts.

[0111] The preparation of multimetal oxide II active compositions isdescribed, for example, in EP-A 575897 and also in DE-A 19855913.

[0112] The inert support materials recommended above are also useful,among other uses, as inert materials for diluting and/or delimiting theparticular fixed catalyst bed or as its guard bed.

[0113] Useful active compositions for the catalysts of the fixedcatalyst bed 2 are in principle all multimetal oxide compositionscomprising Mo and V, for example those of DE-A 10046928.

[0114] A multiplicity of these, for example those of DE-A 19815281, canbe encompassed by the general formula IV

Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(n)  (IV)

[0115] where the variables are defined as follows:

[0116] X¹=W, Nb, Ta, Cr and/or Ce,

[0117] X²=Cu, Ni, Co, Fe, Mn and/or Zn,

[0118] X³=Sb and/or Bi,

[0119] X⁴=one or more alkali metals,

[0120] X⁵=one or more alkaline earth metals,

[0121] X⁶=Si, Al, Ti and/or Zr,

[0122] a=from 1 to 6,

[0123] b=from 0.2 to 4,

[0124] c=from 0.5 to 18,

[0125] d=from 0 to 40,

[0126] e=from 0 to 2,

[0127] f=from 0 to 4,

[0128] g=from 0 to 40 and

[0129] n=a number which is determined by the valency and frequency ofthe elements other than oxygen in IV.

[0130] Embodiments among the active multimetal oxides IV which arepreferred according to the invention are those which are encompassed bythe following definitions of the variables of the general formula IV:

[0131] X¹=W, Nb and/or Cr,

[0132] X²=Cu, Ni, Co and/or Fe,

[0133] X³=Sb

[0134] X⁴=Na and/or K,

[0135] X⁵=Ca, Sr and/or Ba,

[0136] X⁶=Si, Al and/or Ti,

[0137] a=from 1.5 to 5,

[0138] b=from 0.5 to 2,

[0139] c=from 0.5 to 3,

[0140] d=from 0 to 2,

[0141] e=from 0 to 0.2,

[0142] f=from 0 to 1 and

[0143] n=a number which is determined by the valency and frequency ofthe elements other than oxygen in IV.

[0144] However, multimetal oxides IV which are very particularlypreferred according to the invention are those of the general formula V

Mo₁₂V_(a′)Y¹ _(b ′)Y² _(c′)Y⁵ _(f′)Y⁶ _(g′)O_(n′)  (V)

[0145] where

[0146] Y¹=W and/or Nb,

[0147] Y²=Cu and/or Ni,

[0148] Y⁵=Ca and/or Sr,

[0149] Y⁶=Si and/or Al,

[0150] a′=from 2 to 4,

[0151] b′=from 1 to 1.5,

[0152] c′=from 1 to 3,

[0153] f′=from 0 to 0.5

[0154] g′=from 0 to 8 and

[0155] n′=a number which is determined by the valency and frequency ofthe elements other than oxygen in V.

[0156] The multimetal oxide active compositions (IV) which are suitableaccording to the invention are obtainable in a manner known per se, forexample disclosed in DE-A 4335973 or in EP-A 714700.

[0157] In principle, multimetal oxide active compositions, in particularthose of the general formula IV, suitable for the catalysts of the fixedcatalyst bed 2 can be prepared in a simple manner by obtaining a veryintimate, preferably finely divided dry mixture having a compositioncorresponding to their stoichiometry from suitable sources of theirelemental constituents and calcining it at temperatures of from 350 to600° C. The calcination may be carried out either under inert gas orunder an oxidative atmosphere, for example air (mixture of inert gas andoxygen), and also under a reducing atmosphere (for example mixtures ofinert gas and reducing gases such as H₂, NH₃, CO, methane and/oracrolein or the reducing gases mentioned themselves). The calcinationtime can be from a few minutes to a few hours and typically decreaseswith temperature. Useful sources for the elemental constituents of themultimetal oxide active compositions IV include those compounds whichare already oxides and/or those compounds which can be converted tooxides by heating, at least in the presence of oxygen.

[0158] The starting compounds for preparing multimetal oxidecompositions IV can be intimately mixed in dry or in wet form. When theyare mixed in dry form, the starting compounds are advantageously used asfinely divided powder and subjected to calcining after mixing andoptional compaction. However, preference is given to the intimate mixingin wet form.

[0159] This is typically done by mixing the starting compounds in theform of an aqueous solution and/or suspension. Particularly intimate drymixtures are obtained in the mixing process described when the startingmaterials are exclusively sources of the elemental constituents in.dissolved form. The solvent used is preferably water. Subsequently, theaqueous composition obtained is dried, and the drying process ispreferably effected by spray-drying the aqueous mixture at exittemperatures of from 100 to 150° C.

[0160] The resulting multimetal oxide compositions, in particular thoseof the general formula IV, are generally used in the fixed catalyst bed2 not in powder form, but rather shaped to certain catalyst geometries,and the shaping may be effected before or after the final calcination.For example, unsupported catalysts can be prepared from the powder formof the active composition or its uncalcined precursor composition bycompacting to the desired catalyst geometry (for example by tableting orextruding), optionally with the addition of assistants, for examplegraphite or stearic acid as lubricants and/or shaping assistants andreinforcing agents such as microfibers of glass, asbestos, siliconcarbide or potassium titanate. Examples of suitable unsupported catalystgeometries are solid cylinders or hollow cylinders having an externaldiameter and a length of from 2 to 10 mm. In the case of the hollowcylinder, a wall thickness of from 1 to 3 mm is advantageous. It will beappreciated that the unsupported catalyst may also have sphericalgeometry and the spherical diameter may be from 2 to 10 mm.

[0161] It will be appreciated that the pulverulent active composition orits pulverulent precursor composition which is yet to be calcined canalso be shaped by applying to preshaped inert catalyst supports. Thecoating of the support bodies to prepare the coated catalysts isgenerally performed in a rotatable vessel, as disclosed, for example, byDE-A 2909671, EP-A 293859 or from EP-A 714700.

[0162] To coat the support bodies, the powder composition to be appliedis advantageously moistened and is dried again after application, forexample by means of hot air. The coating thickness of the powdercomposition applied to the support body is advantageously selectedwithin the range from 10 to 1 000 μm, preferably within the range from50 to 500 μm and more preferably in the range from 150 to 200 μm.

[0163] Useful support materials are customary porous or nonporousaluminum oxides, silicon dioxide, thorium dioxide, zirconium dioxide,silicon carbide or silicates such as magnesium silicate or aluminumsilicate. The support bodies may have a regular or irregular shape,although preference is given to regularly shaped support bodies havingdistinct surface roughness, for example spheres or hollow cylindershaving a grit layer. It is suitable to use substantially nonporous,surface-roughened, spherical supports made of steatite (e.g. SteatiteC220 from CeramTec) whose diameter is from 1 to 8 mm, preferably from 4to 5 mm. However, suitable support bodies also include cylinders whoselength is from 2 to 10 mm and whose external diameter is from 4 to 10mm. In the case of rings as support bodies, the wall thickness is alsotypically from 1 to 4 mm. Annular support bodies to be used withpreference have a length of from 2 to 6 mm, an external diameter of from4 to 8 mm and a wall thickness of from 1 to 2 mm. Suitable supportbodies are also in particular rings of geometry 7 mm×3 mm×4 mm (externaldiameter×length×internal diameter). It will be appreciated that thefineness of the catalytically active oxide compositions to be applied tothe surface of the support body is adapted to the desired coatingthickness (cf. EP-A 714 700).

[0164] Advantageous multimetal oxide active compositions to be used forthe catalysts of the fixed catalyst bed 2 are also compositions of thegeneral formula VI

[D]_(p)[E]_(q)  (VI)

[0165] where the variables are defined as follows:

[0166] D=Mo₁₂V_(a″)Z¹ _(b″)Z² _(c″)Z³ _(d″)Z⁴ _(e″)Z⁵ _(f″)Z⁶_(g″)O_(x″),

[0167] E=Z⁷ ₁₂Cu_(h″)H_(i″)O_(″),

[0168] Z¹=W, Nb, Ta, Cr and/or Ce,

[0169] Z²=Cu, Ni, Co, Fe, Mn and/or Zn,

[0170] Z³=Sb and/or Bi,

[0171] Z⁴=Li, Na, K, Rb, Cs and/or H,

[0172] Z⁵=Mg, Ca, Sr and/or Ba,

[0173] Z⁶=Si, Al, Ti and/or Zr,

[0174] Z⁷=Mo, W, V, Nb and/or Ta, preferably Mo and/or W

[0175] a″=from 1 to 8,

[0176] b″=from 0.2 to 5,

[0177] c″=from 0 to 23,

[0178] d″=from 0 to 50,

[0179] e″=from 0 to 2,

[0180] f′=from 0 to 5,

[0181] g″=from 0 to 50,

[0182] h″=from 4 to 30,

[0183] i″=from 0 to 20 and

[0184] x″,y″=numbers which are determined by the valency and frequencyof the elements other than oxygen in VI and

[0185] p,q=numbers other than zero whose p/q ratio is from 160:1 to 1:1,

[0186] and which are obtainable by separately preforming a multimetaloxide composition E

Z⁷ ₁₂Cu_(h″)H_(i″)O_(y″)  (E)

[0187] in finely divided form (starting composition 1) and subsequentlyincorporating the preformed solid starting composition 1 into an aqueoussolution, an aqueous suspension or into a finely divided dry mixture ofsources of the elements Mo, V, Z¹, Z², Z³, Z⁴, Z⁵, Z⁶ which comprisesthe abovementioned elements in the stoichiometry D

[0188] Mo¹²V^(a″)Z¹ _(b″)Z² _(c″)Z³ _(d″)Z⁴ _(e″)Z⁵ _(f″)Z⁶ _(g″)  (D)

[0189] (starting composition 2) in the desired p:q ratio, drying theaqueous mixture which may result, and calcining the resulting dryprecursor composition before or after drying at temperatures of from 250to 600° C. to give the desired catalyst geometry.

[0190] Preference is given to the multimetal oxide compositions VI inwhich the preformed solid starting composition 1 is incorporated into anaqueous starting composition 2 at a temperature of <70° C. A detaileddescription of the preparation of multimetal oxide VI catalysts iscontained, for example, in EP-A 668104, DE-A 19736105, DE-A 10046928,DE-A 19740493 and DE-A 19528646.

[0191] With regard to the shaping, the statements made for themultimetal oxide IV catalysts apply to the multimetal oxide VIcatalysts.

[0192] Further suitable multimetal oxide compositions for the catalystsof the fixed catalyst bed 2 are those of DE-A 19815281, in particularall exemplary embodiments from this document. With regard to theshaping, the same applies as was stated above. For the fixed catalystbed 2 of the process according to the invention, particularly suitablecatalysts are the coated catalysts S1 (stoichiometry:Mo₁₂V₃W_(1.2)Cu_(2.4)O_(n)) and S7 (stoichiometry:Mo₁₂V₃W_(1.2)Cu_(1.6)Ni_(0.8)O_(n)) from DE-A 4442346 having an activecomposition fraction of 27% by weight and a coating thickness of 230 μm,the coated catalyst from preparative example 5 of DE-A 10046928(stoichiometry: Mo₁₂V₃W_(1.2)Cu_(2.4)O_(n)) having an active compositionfraction of 20% by weight, to coated catalysts of examples 1 to 5 fromDE-A 19815281, but equally the abovementioned coated catalysts for thesecond reaction stage applied to support rings of geometry 7 mm×3 mm×4mm (external diameter×length×internal diameter) having an activecomposition fraction of 20% by weight (based on the overall compositionof the coated catalyst), and also a coated catalyst having a biphasicactive composition of stoichiometry (Mo_(10.4)V₃W_(1.2)O_(x))(CuMo_(0.5)W_(0.5)O₄)_(1.6) and prepared according A 19736105 and havingan active composition fraction of 20% by weight applied to theabovementioned 7 mm×3 mm×4 mm support.

[0193] The catalysts recommended above for the second reaction stage areonly suitable for the two reaction stages when everything is retainedexcept the support geometry which is changed to 5 mm×3 mm×1.5 mm(external diameter×length×internal diameter). The multimetal oxidesmentioned can also be used in the second reaction stage in the form ofthe corresponding unsupported catalyst rings.

[0194] To prepare the fixed catalyst bed 1 in the process according tothe invention, it is possible to use only the shaped catalyst bodieshaving the appropriate multimetal oxide active composition or elsesubstantially homogeneous mixtures of shaped catalyst bodies havingmultimetal oxide active composition and shaped bodies (shaped diluentbodies) behaving substantially inertly with regard to theheterogeneously catalyzed partial gas phase oxidation and having nomultimetal oxide active composition. Useful materials for such inertshaped bodies include in principle all of those which are also suitableas support material for coated catalysts suitable in accordance with theinvention. Useful such materials include, for example, porous andnonporous aluminum oxides, silicon dioxide, thorium dioxide, zirconiumdioxide, silicon carbide, silicates such as magnesium silicate oraluminum silicate or the steatite already mentioned (e.g. Steatite C-220from CeramTec).

[0195] The geometry of such inert shaped diluent bodies may in principlebe as desired. In other words, they may, for example, be spheres,polygons, solid cylinders or else rings. According to the invention, theinert shaped diluent bodies selected will preferably be those whosegeometry corresponds to that of the shaped catalyst bodies to be dilutedby them (the above statements also apply in the same way to thesubstantially homogeneous mixtures of shaped catalyst bodies havingappropriate multimetal oxide active composition and shaped diluentbodies which can be used to provide the fixed catalyst bed 2).

[0196] According to the invention, it is advantageous when the chemicalcomposition of the active composition used does not change over thefixed catalyst bed 1. In other words, although the active compositionused for an individual shaped catalyst body can be a mixture ofdifferent multimetal oxides containing the elements Mo, Fe and Bi, thesame mixture then has to be used for all shaped catalyst bodies of thefixed catalyst bed 1.

[0197] In this case, the volume-specific (i.e. normalized to the unit ofthe volume) activity can be reduced in a simple manner, for example, byhomogeneously diluting a basic amount of shaped catalyst bodies preparedin a uniform manner with shaped diluent bodies. The higher theproportion of shaped diluent bodies selected, the less activecomposition and catalyst activity present in a certain volume of thebed.

[0198] A volume-specific activity increasing at least once in the flowdirection of the reaction gas mixture over the fixed catalyst bed 1 cantherefore be attained for the process according to the invention in asimple manner, for example, by beginning the bed with a high proportionof inert shaped diluent bodies based on one type of shaped catalystbodies, and then, either continuously or at least once or more than onceabruptly (for example stepwise), reducing this proportion of shapeddiluent bodies in the flow direction. When the proportion of shapeddiluent bodies is left constant or no shaped diluent bodies at all areused in the fixed catalyst bed 1, this then results in a constantvolume-specific activity in the flow direction of the reaction gasmixture over the fixed catalyst bed 1. However, an increase in thevolume-specific activity is also possible, for example, at a constantgeometry and active composition type of a shaped coated catalyst body,increasing the thickness of the active composition layer applied to thissupport, or, in a mixture of coated catalysts having the same geometrybut having different proportions by weight of the active composition,increasing the proportion of shaped catalyst bodies having the higherproportion by weight of active composition. Alternatively, the activecompositions themselves can also be diluted in the course of the activecomposition preparation by, for example, incorporating inert, dilutingmaterials such as hard-fired silica into the dry mixture of startingcompounds to be calcined. Different amounts of diluting material addedlead automatically to different activities. The more diluting materialis added, the lower the resulting activity will be. A similar effect canalso be achieved, for example, in mixtures of unsupported catalysts andof coated catalysts (having identical active composition) by varying themixing ratio in an appropriate manner. It will be appreciated that thevariants described can also be used in combination.

[0199] It is of course also possible to use mixtures of catalysts havingchemically different active compositions and, as a consequence of thisdifferent composition, having different activity for the fixed catalystbed 1. These mixtures may in turn be diluted with inert shaped diluentbodies.

[0200] Normally, the volume-specific activity in the process accordingto the invention will decrease not once either within the fixed catalystbed 1 or within the fixed catalyst bed 2 in the flow direction of thereaction gas mixture.

[0201] Upstream and/or downstream of the fixed catalyst bed 1 may bedisposed beds consisting exclusively of inert material (for example onlyshaped diluent bodies) (in this document, they are not included forterminology purposes in the fixed catalyst bed 1, since they contain noshaped bodies which have multimetal oxide active composition). Theshaped diluent bodies used for the inert bed can have the same geometryas the shaped catalyst bodies used in the fixed catalyst bed 1. However,the geometry of the shaped diluent bodies used for the inert bed canalso be different to the abovementioned geometry of the shaped catalystbodies (for example, spherical instead of annular).

[0202] Frequently, the shaped bodies used for such inert beds have theannular geometry 7 mm×7 mm×4 mm (external diameter×length×internaldiameter) or the spherical geometry having the diameter d=4-5 mm.

[0203] According to the invention, preference is given in the processaccording to the invention to the fixed catalyst bed 1 being structuredin the flow direction of the reaction gas mixture as follows.

[0204] First, to a length of from 10 to 60%, preferably from 10 to 50%,more preferably from 20 to 40% and most preferably from 25 to 35% (i.e.,for example, to a length of from 0.70 to 1.50 m, preferably from 0.90 to1.20 m), each of the total length of the fixed catalyst bed 1, ahomogeneous mixture of shaped catalyst bodies and shaped diluent bodies(both preferably having substantially the same geometry), in which theproportion by weight of shaped diluent bodies (the densities of shapedcatalyst bodies and of shaped diluent bodies generally differ onlyslightly) is normally from 5 to 40% by weight, or from 10 to 40% byweight, or from 20 to 40% by weight, or from 25 to 35% by weight.According to the invention, this first zone of the fixed catalyst bed 1is advantageously followed to the end of the length of the fixedcatalyst bed 1 (i.e., for example, to a length of from 2.00 to 3.00 m,preferably from 2.50 to 3.00 m) either by a bed of shaped catalystbodies diluted only to a slighter extent (than in the first zone), or,most preferably, an unaccompanied (undiluted) bed of the same shapedcatalyst bodies which have also been used in the first zone. Theaforementioned applies in particular when the shaped catalyst bodiesused in the fixed catalyst bed 1 are unsupported catalyst rings orcoated catalyst rings (in particular those which are listed in thisdocument as preferred). For the purposes of the aforementionedstructuring, both the shaped catalyst bodies and the shaped diluentbodies in the process according to the invention substantially have thering geometry 5 mm×3 mm×2 mm (external diameter×length×internaldiameter).

[0205] In a corresponding manner to that in which the volume-specificactivity of the fixed catalyst bed 1 can be varied, the volume-specificactivity of the fixed catalyst bed 2 can also be varied. Upstream and/ordownstream of the actual fixed catalyst bed 2 may again be disposed anappropriate inert bed. However, a constant volume-specific activitywithin the fixed catalyst bed 2 (as is possible in accordance with theinvention within the fixed catalyst bed 1) is ruled out in the processaccording to the invention.

[0206] Preference is given in accordance with the invention to the fixedcatalyst bed 2 in the process according to the invention beingstructured as follows in the flow direction of the reaction gas mixture.

[0207] First, to a length of from 10 to 60%, preferably from 10 to 50%,more preferably from 20 to 40% and most preferably from 25 to 35% (i.e.,for example, to a length of from 0.70 to 1.50 m, preferably from 0.90 to1.20 m), each of the total length of the fixed bed catalyst bed 2, ahomogeneous mixture or two (having decreasing dilution) successivehomogeneous mixtures of shaped catalyst bodies and shaped diluent bodies(both preferably having substantially the same geometry), in which theproportion by weight of shaped diluent bodies (the densities of shapedcatalyst bodies and of shaped diluent bodies generally differ onlyslightly) is normally from 10 to 50% by weight, preferably from 20 to45% by weight and more preferably from 25 to 35% by weight. According tothe invention, this first zone of fixed catalyst bed 2 is thenadvantageously followed to the end of the length of the fixed catalystbed 2 (i.e., for example, to a length of from 2.00 to 3.00 m, preferablyfrom 2.50 to 3.00 m) by either a bed of the shaped catalyst bodiesdiluted only to a slighter extent (than in the first zone) or, mostpreferably, an unaccompanied bed of the same shaped catalyst bodieswhich have also been used in the first zone.

[0208] The aforementioned applies in particular when the shaped catalystbodies used in the fixed catalyst bed 2 are coated catalyst rings orcoated catalyst spheres (in particular those which are listed in thisdocument as preferred). For the purposes of the aforementionedstructuring, both the shaped catalyst bodies or their support rings andthe shaped diluent bodies in the process according to the inventionsubstantially have the annular geometry 7 mm×3 mm×4 mm (externaldiameter×length×internal diameter).

[0209] The abovementioned also applies when, instead of inert shapeddiluent bodies, shaped coated catalyst bodies are used as activecomposition content is from 2 to 15% by weight lower than the activecomposition content of the shaped coated catalyst bodies at the end ofthe fixed catalyst bed 2.

[0210] In an advantageous manner from an application point of view, thefirst reaction stage of the process according to the invention iscarried out in a two-zone tube bundle reactor, as described, forexample, in DE-A 19910508, 19948523, 19910506 and 19948241. A preferredvariant of a two-zone tube bundle reactor which can be used inaccordance with the invention is disclosed by DE-C 2830765. However, thetwo-zone tube bundle reactors disclosed in DE-C 2513405, U.S. Pat. No.3147084, DE-A 2201528, EP-A 383224 and DE-A 2903218 are also suitablefor carrying out the first reaction stage of the process according tothe invention.

[0211] In other words, in the simplest manner, the fixed catalyst bed 1to be used in accordance with the invention (possibly with downstreamand/or upstream inert beds) is disposed in the metal tubes of a tubebundle reactor and two spatially separated heating media, generally saltmelts, are conducted around the metal tubes. The tube section over whichthe particular salt bath extends represents a reaction zone inaccordance with the invention. In other words, in the simplest manner,for example, a salt bath A flows around that section of the tubes(reaction zone A) in which propene is oxidatively converted (on singlepass) until a conversion in the range from 40 to 80 mol % is achieved,and a salt bath B flows around the section of the tubes (reaction zoneB) in which the propene is subsequently oxidatively converted (on singlepass) until a conversion value of at least 90 mol % is achieved (ifrequired, the reaction zones A, B to be used in accordance with theinvention can be followed by further reaction zones which are maintainedat individual temperatures).

[0212] It is advantageous from an application point of view if the firstreaction stage of the process according to the invention includes nofurther reaction zones. In other words, the salt bath B advantageouslyflows around the section of the tubes in which propene is subsequentlyoxidatively converted (on single pass) up to a conversion value of ≧90mol %, or ≧92 mol %, of ≧94 mol % or more.

[0213] Typically, the beginning of the temperature zone B lies beyondthe heating point maximum of temperature zone A.

[0214] According to the invention, both salt baths A, B can be conductedin cocurrent or in countercurrent through the space surrounding thereaction tubes relative to the flow direction of the reaction gasmixture flowing through the reaction tubes. It will be appreciated that,in accordance with the invention, cocurrent flow may be applied inreaction zone A and countercurrent flow in reaction zone B (or viceversa).

[0215] In all of the aforementioned cases, it will be appreciated that atransverse flow can be superimposed on the parallel flow of the saltmelt relative to the reaction tubes taking place within the particularreaction zone, so that the individual reaction zone corresponds to atube bundle reactor as described in EP-A 700714 or in EP-A 700893, whichresults overall in a meandering flow profile of the heat exchange mediumin a longitudinal section through the catalyst tube bundle.

[0216] Advantageously, the starting reaction gas mixture 1 in theprocess according to the invention is fed to the fixed catalyst bed 1preheated to the reaction temperature.

[0217] Typically, the catalyst tubes in the two-zone tube bundlereactors are manufactured from ferritic steel and typically have a wallthickness of from 1 to 3 mm. Their internal diameter is generally from20 to 30 mm, frequently from 21 to 26 mm. Their length is advantageouslyfrom 2 to 4 m, preferably from 2.5 to 3.5 m. In each temperature zone,the fixed catalyst bed 1 occupies at least 60%, or at least 75%, or atleast 90%, of the length of the zone. Any remaining length is optionallyoccupied by an inert bed. It is advantageous from an application pointof view for the number of catalyst tubes accommodated in the tube bundlevessel to be at least 5 000, preferably at least 10 000. Frequently, thenumber of catalyst tubes accommodated in the reaction vessel is from 15000 to 30 000. Tube bundle reactors having a number of catalyst tubesabove 40 000 are usually exceptional. Within the vessel, the catalysttubes are normally homogeneously distributed (preferably 6 equidistantadjacent tubes per catalyst tube), and the distribution isadvantageously selected in such a way that the separation of the centralinternal axes of immediately adjacent catalyst tubes (the catalyst tubepitch) is from 35 to 45 mm (cf., for example, EP-B 468290).

[0218] Useful heat exchange media for the two-zone method are also inparticular fluid heating media. It is particularly advantageous to usemelts of salts such as potassium nitrate, potassium nitrite, sodiumnitrite and/or sodium nitrate, or of low-melting metals such as sodium,mercury and also alloys of different metals.

[0219] In general, in all of the aforementioned flow arrangements in thetwo-zone tube bundle reactors, the flow rate within the two heatexchange medium circuits required is selected in such a way that thetemperature of the heat exchange medium rises from the entrance into thereaction zone to the exit from the reaction zone (as a result of theexothermicity of the reaction) by from 0 to 15° C. In other words, theaforementioned ΔT may be from 1 to 10° C., or from 2 to 8° C., or from 3to 6° C., in accordance with the invention.

[0220] According to the invention, the entrance temperature of the heatexchange medium into reaction zone A is normally in the range from 290to 380° C., preferably in the range from 305 to 365° C. and morepreferably in the range from 310 to 340° C. or is 330° C.

[0221] According to the invention, the entrance temperature of the heatexchange medium into reaction zone B is normally likewise in the rangefrom 290 to 380° C., but at the same time normally from ≧0° C. to ≦20°C. or ≦10° C., or ≧0° C. and ≦5° C., or frequently ≧0° C. and ≦3° C.,below the entrance temperature of the heat exchange medium enteringreaction zone A.

[0222] It is pointed out once again at this juncture that, for animplementation of reaction stage 1 of the process according to theinvention, it is possible to use in particular the two-zone tube bundlereactor type described in DE-B 2201528 which includes the possibility ofremoving the hotter heat exchange medium of reaction zone B to reactionzone A, in order to optionally heat a cold starting reaction gas mixtureor a cold cycle gas. The tube bundle characteristics within anindividual reaction zone may also be configured as described in EP-A382098.

[0223] It has proven advantageous in accordance with the invention tocool the product gas mixture leaving the first reaction stage beforeentering into the second reaction stage in a direct and/or indirectmanner, in order to suppress subsequent full combustion of portions ofthe acrolein formed in the first reaction stage. To this end, anaftercooler is arranged between the two reaction stages. In the simplestcase, this may be an indirect tube bundle heat exchanger. In this case,the product gas mixture is generally conducted through the tubes and aheat exchange medium is conducted around the tubes and may be of thetype corresponding to the heat exchange media recommended for the tubebundle reactors. Advantageously, the tube interior is filled with inertrandom packings (for example spirals of stainless steel, rings ofsteatite, spheres of steatite, etc.). These improve the heat exchangeand capture any molybdenum trioxide subliming from the fixed catalystbed of the first reaction stage before its entry into the secondreaction stage. It is advantageous if the aftercooler is manufacturedfrom stainless steel coated with zinc silicate primer.

[0224] In general, the propene conversion based on single pass in thefirst reaction stage of the process according to the invention will be≧92 mol % or ≧94 mol %. According to the invention, the resultingselectivity in the first reaction stage of acrolein formation and alsoof acrylic acid by-production together on single pass will regularly be≧92 mol % or ≧94 mol %, frequently ≧95 mol % or ≧96 mol % or ≧97 mol %.

[0225] Useful sources for the molecular oxygen required in the firstreaction stage include both air and air depleted of molecular nitrogen(for example, ≧90 by volume of O₂, ≦10% by volume of N₂).

[0226] It is advantageous from an application point of view to cool theproduct gas mixture of the first reaction stage in the aftercooleralready mentioned to a temperature of from 210 to 290° C., frequentlyfrom 230 to 280° C. or from 250 to 270° C. The product gas mixture ofthe first reaction stage can quite possibly be cooled to temperatureswhich are below the temperature of the second reaction stage. However,the aftercooling described is no way obligatory and can generally bedispensed with, especially when the path of the product gas mixture fromthe first reaction stage to the second reaction stage is kept short.Typically, the process according to the invention is also realized insuch a way that the oxygen requirement in the second reaction stage isnot already covered by an appropriately high oxygen content of thestarting reaction gas mixture 1, but rather that the required oxygen isadded in the region between the first and second reaction stages(“secondary gas addition”). This can be before, during, after and/or foraftercooling. Useful sources for the molecular oxygen required in thesecond reaction stage include both pure oxygen and mixtures of oxygenand inert gas, for example air or air depleted of molecular nitrogen(for example, ≧90% by volume of O₂, ≦10% by volume of N₂). The oxygensource is regularly added compressed to the reaction pressure. It willbe appreciated that the oxygen requirement in the second reaction stageof the process according to the invention can be covered by anappropriately high oxygen requirement in the first reaction stage. Ifrequired, an inert diluent gas can of course also be added as asecondary gas.

[0227] Like the implementation of the first reaction stage, the secondreaction stage of the process according to the invention is alsoimplemented in an advantageous manner from an application point of viewin a two-zone tube bundle reactor, as already described for the firstreaction stage. The remarks regarding two-zone tube bundle reactor forthe first reaction stage therefore also apply to the two-zone tubebundle reactor for the second reaction stage.

[0228] In other words, the fixed catalyst bed 2 (including any inertbeds) to be used in accordance with the invention is disposed in asimple manner in the metal tubes of a tube bundle reactor and twosubstantially spatially separated heating media, generally salt melts,are conducted around the metal tubes. According to the invention, thetube section over which the respective salt bath extends represents areaction zone.

[0229] In other words, for example, a salt bath C flows around thosesections of the tubes (reaction zone C) in which the acrolein isoxidatively converted (on single pass) until a conversion value in therange from 45 to 85 mol % (preferably from 50 to 85 mol %, morepreferably from 60 to 85 mol %) is achieved, and a salt bath D flowsaround the section of the tubes (reaction zone D) in which the acroleinis subsequently oxidatively converted (on single pass) until aconversion value of at least 90 mol % is achieved (if required, thereaction zones C, D to be used in accordance with the invention can befollowed by further reaction zones which are maintained at individualtemperatures).

[0230] It is advantageous in accordance with the invention if thereaction stage 2 of the process according to the invention includes nofurther reaction zones. In other words, the salt bath D advantageouslyflows around the section of the tubes in which the acrolein issubsequently oxidatively converted (on single pass) to a conversionvalue of ≧92 mol %, or ≧94 mol % or ≧96 mol % or ≧98 mol % andfrequently even ≧99 mol % or more.

[0231] Typically, the beginning of the reaction zone D lies beyond theheating point maximum of reaction zone C.

[0232] According to the invention, both salt baths C, D can be conductedin cocurrent or in countercurrent through the space surrounding thereaction tubes relative to the flow direction of the reaction gasmixture flowing through the reaction tubes. It will be appreciated that,in accordance with the invention, cocurrent flow may be applied inreaction zone C and countercurrent flow in reaction zone D (or viceversa).

[0233] In all of the aforementioned cases, it will be appreciated that atransverse flow can be superimposed on the parallel flow of the saltmelt relative to the reaction tubes taking place within the particularreaction zone, so that the individual reaction zone corresponds to atube bundle reactor as described in EP-A 700714 or in EP-A 700893, whichresults overall in a meandering flow profile of the heat exchange mediumin a longitudinal section through the catalyst tube bundle.

[0234] Typically, the catalyst tubes in the aforementioned two-zonebundle reactors for the second reaction stage are manufactured fromferritic steel and typically have a wall thickness of from 1 to 3 mm.Their internal diameter is generally from 20 to 30 mm, frequently from21 to 26 mm. Their length is advantageously from 3 to 4 m, preferably3.5 m. In each temperature zone, the fixed catalyst bed 2 occupies atleast 60%, or at least 75%, or at least 90%, of the length of the zone.Any remaining length is optionally occupied by an inert bed. It isadvantageous from an application point of view for the number ofcatalyst tubes accommodated in the tube bundle vessel to be at least 5000, preferably at least 10 000. Frequently, the number of catalysttubes accommodated in the reaction vessel is from 15 000 to 30 000. Tubebundle reactors having a number of catalyst tubes above 40 000 areusually exceptional. Within the vessel, the catalyst tubes are normallyhomogeneously distributed (preferably 6 equidistant adjacent tubes percatalyst tube), and the distribution is advantageously selected in sucha way that the separation of the central internal axes of immediatelyadjacent catalyst tubes (the catalyst tube pitch) is from 35 to 45 mm(cf. EP-B 468290).

[0235] Useful heat exchange media are in particular fluid heating media.It is particularly advantageous to use melts of salts such as potassiumnitrate, potassium nitrite, sodium nitrite and/or sodium nitrate, or oflow-melting metals such as sodium, mercury and also alloys of differentmetals.

[0236] In general, in all of the abovementioned flow arrangements in thetwo-zone tube bundle reactors of the second reaction stage, the flowrate within the two heat exchange medium circuits required is selectedin such a way that the temperature of the heat exchange medium risesfrom the entrance into the reaction zone to the exit from the reactionzone by from 0 to 15° C. In other words, the aforementioned ΔT may befrom 1 to 10° C., or from 2 to 8° C., or from 3 to 6° C., in accordancewith the invention.

[0237] According to the invention, the entrance temperature of the heatexchange medium into reaction zone C is normally in the range from 230to 320° C., preferably in the range from 250 to 300° C. and morepreferably in the range from 260 to 280° C. According to the invention,the entrance temperature of the heat exchange medium into reaction zoneD is normally likewise in the range from 230 to 280° C., but at the sametime normally from ≧0° C. to ≦20° C. or ≦10° C., or ≧0° C. and ≦5° C.,or frequently ≧0° C. and ≦3° C., below the entrance temperature of theheat exchange medium entering reaction zone C.

[0238] It is pointed out once again at this juncture that, for animplementation of the second reaction stage of the process according tothe invention, it is possible to use in particular the two-zone tubebundle reactor type described in DE-B 2201528 which includes thepossibility of removing the hotter heat exchange medium of reaction zoneD to reaction zone C, in order to optionally heat a cold startingreaction gas mixture 2 or a cold cycle gas. The tube bundlecharacteristics within an individual reaction zone may also beconfigured as described in EP-A 382098.

[0239] It will be appreciated that the process according to theinvention can also be carried out by combining two two-zone tube bundlereactors to give a four-zone tube bundle reactor, as described in WO01/36364. In these cases, there is normally an inert bed between thefixed catalyst bed 1 and the fixed catalyst bed 2. However, such anintermediate inert bed may also be dispensed with. The length of thereaction tubes in the event of combination corresponds in many cases tothe sum of the lengths of the uncombined tube bundle reactors.

[0240] The propene content in the starting reaction gas mixture 1 in theprocess according to the invention can, for example, be at values offrom 4 to 15% by volume, frequently from 5 to 12% by volume, or from 5to 8% by volume (based in each case on the total volume).

[0241] Frequently, the process according to the invention will becarried out at a propene:oxygen:inert gases (including steam) volumeratio in the starting reaction gas mixture 1 of 1:(1.0 to 3.0):(5 to25), preferably 1:(1.7 to 2.3):(10 to 15). In general, at least 20% ofthe volume of the inert gas will consist of molecular nitrogen. However,≧30% by volume, or ≧40% by volume, or ≧50% by volume, or ≧60% by volume,or ≧70% by volume, or ≧80% by volume, or ≧90% by volume, or ≧95% byvolume, of the inert gas may consist of molecular nitrogen (in thisdocument, inert diluent gases generally refer to those of which lessthan 5%, preferably less than 2%, is converted in single pass throughthe particular reaction stage; in addition to molecular nitrogen, theseare, for example, gases such as propane, ethane, methane, pentane,butane, CO₂, CO, steam and/or noble gases). Up to 50 mol % or up to 75mol % and more of the inert diluent gas in the process according to theinvention can consist of propane. Cycle gas, as remains after theremoval of the acrylic acid from the product gas mixture, can also be aconstituent of the diluent gas.

[0242] The aforementioned composition ranges are applied both in casesof secondary gas feed and in cases where no secondary gas is fed.

[0243] The starting reaction gas mixtures 1 which are advantageousaccording to the invention are, for example, those which are composed of

[0244] from 6 to 15 (preferably from 7 to 11) % by volume of propene,

[0245] from 4 to 20 (preferably from 6 to 12) % by volume of water,

[0246] from ≧0 to 10 (preferably from ≧0 to 5) % by volume ofconstituents other than propene, water, oxygen and nitrogen,

[0247] sufficient molar oxygen that the molar ratio of molar oxygenpresent to propene present is from 1.5 to 2.5 (preferably from 1.6 to2.2), and the remainder up to 100% by volume of the total amount ofmolecular nitrogen,

[0248] as recommended by DE-A 10302715.

[0249] According to the invention, the acrolein content in the startingreaction gas mixture 2 can have, for example, values of from 3 to 15% byvolume, frequently from 4 to 10% by volume or from 5 to 8% by volume(based in each case on the total volume).

[0250] Frequently, the process according to the invention will beperformed at an acrolein:oxygen:steam:inert gas volume ratio (l (STP))present in the starting reaction gas mixture 2 of 1:(1 to 3):(0 to20):(3 to 30), preferably of 1:(1 to 3):(0.5 to 10):(7 to 10).

[0251] It will be appreciated that the process according to theinvention can also be carried out at an acrolein:oxygen:steam:othersvolume ratio (l (STP)) of 1:(0.9 to 1.3):(2.5 to 3.5):(10 to 12).

[0252] It is emphasized at this juncture that the multimetal oxidecompositions of DE-A 10261186 are advantageous active compositions bothfor the fixed bed catalyst 1 and for the fixed catalyst bed 2.

[0253] Designs of a two-zone tube bundle reactor for the first reactionstage which are advantageous in accordance with the invention can havethe following construction (the detailed configuration of theconstruction can be as described in the utility model applications 20219 277.6, 2002 19 278.4 and 202 19 279.2 or in the PCT applicationsPCT/EP02/14187, PCT/EP02/14188 or PCT/EP02/14189):

[0254] Catalyst tubes:

[0255] material of the catalyst tubes: ferritic steel;

[0256] dimensions of the catalyst tubes: length, for example, 3 500 mm;external diameter, for example, 30 mm; wall thickness, for example, 2mm; number of catalyst tubes in the tube bundle: for example, 30 000, or28 000, or 32 000, or 34 000; in addition up to 10 thermal tubes (asdescribed in EP-A 873 783 and EP-A 12 70 065) which are charged in thesame way as the catalyst tubes (in a spiral manner rotating from thevery outside toward the inside), for example of the same length and wallthickness but having an external diameter of, for example, 33.4 mm and acentered thermowell of external diameter of, for example, 8 mm and wallthickness of, for example, 1 mm;

[0257] reactor (same material as the catalyst tubes):

[0258] cylindrical vessel of internal diameter 6 000-8 000 mm;

[0259] reactor hoods plated with stainless steel of the type 1.4541;plating thickness: a few mm;

[0260] annularly arranged tube bundle, for example with free centralspace: diameter of the free central space: for example, 1 000-2 500 mm(for example 1 200 mm, or 1 400 mm, or 1 600 mm, or 1 800 mm, or 2 000mm, or 2 200 mm, or 2 400 mm);

[0261] normally homogeneous catalyst tube distribution in the tubebundle (6 equidistant adjacent tubes per catalyst tube), arrangement inan equilateral triangle, catalyst tube pitch (separation of the centralinternal axes of immediately adjacent catalyst tubes): 35-45 mm, forexample 36 mm, or 38 mm, or 40 mm, or 42 mm, or 44 mm;

[0262] the catalyst tubes are secured and sealed by their ends incatalyst tube plates (upper plate and lower plate each having athickness, for example, of 100-200 mm) and open at their upper ends intoa hood joined to the vessel which has an inlet for the starting reactiongas mixture; a separating plate of thickness 20-100 mm disposed, forexample, at half the catalyst tube length, divides the reactor spacesymmetrically into two reaction zones (temperature zones) A (upper zone)and B (lower zone); each reaction zone is divided into 2 equidistantlongitudinal sections by deflecting plates;

[0263] the deflecting plate preferably has annular geometry; thecatalyst tubes are advantageously secured and sealed at the separatingplate; they are not secured and sealed at the deflecting plates, so thatthe transverse flow rate of the salt melt within one zone is veryconstant;

[0264] each zone is provided with salt melt as a heat carrier by its ownsalt pump; the feed of the salt melt is, for example, below thedeflecting plate and the withdrawal is, for example, above thedeflecting plate;

[0265] a substream is, for example, removed from both salt melt circuitsand cooled, for example, in one common or two separate indirect heatexchangers (steam generation); in the first case, the cooled salt meltstream is divided, combined with the particular residual stream andpressurized into the reactor by the particular pump into the appropriateannular channel which divides the salt melt over the circumference ofthe vessel;

[0266] the salt melt reaches the tube bundle through the window disposedin the reactor jacket; the flow is, for example, in a radial directionto the tube bundle;

[0267] in each zone, the salt melt flows around the catalyst tubes asdictated by the deflection plate, for example in the sequence

[0268] from the outside inward,

[0269] from the inside outward;

[0270] the salt melt flows through a window mounted around thecircumference of the vessel and collects at the end of each zone in anannular channel disposed around the reactor jacket, in order to bepumped in a circuit including substream cooling;

[0271] the salt melt is conducted from bottom to top through eachtemperature zone.

[0272] The reaction gas mixture leaves the reactor of the first stage ata temperature a few degrees higher than the salt bath entrancetemperature of the first reactor. For further processing, the reactiongas mixture is advantageously cooled to from 220° C. to 280° C.,preferably from 240° C. to 260° C., in a separate aftercooler which isconnected downstream of the reactor of the first stage.

[0273] The aftercooler is generally flanged on below the lower tubeplate and normally consists of tubes of ferritic steel. Stainless steelsheet metal spirals which may be partly or fully wound areadvantageously introduced into the interior of the tubes of theaftercooler, in order to improve the heat transfer.

[0274] Salt Melt:

[0275] The salt melt used may be a mixture of 53% by weight of potassiumnitrate, 40% by weight of sodium nitrite and 7% by weight of sodiumnitrate; both reaction zones and the aftercooler advantageously use asalt melt of the same composition; the amount of salt pumped bycirculation in the reaction zones may be approx. 10 000 m³/h per zone.

[0276] Flow Control:

[0277] The starting reaction gas mixture 1 advantageously flows from topto bottom through the first stage reactor, while the salt melts havingdifferent temperatures of the individual zones are advantageouslyconveyed from bottom to top;

[0278] Catalyst tube and thermal tube charge (from top to bottom), forexample:

[0279] Section 1: length 50 cm steatite rings of geometry 7 mm×7 mm×4 mm(external diameter×length×internal diameter) as a preliminary bed.

[0280] Section 2: length 140 cm catalyst charge of a homogeneous mixtureof 30% by weight of steatite rings of geometry 5 mm×3 mm×2 mm (externaldiameter×length×internal diameter) and 70% by weight of unsupportedcatalyst from section 3.

[0281] Section 3: length 160 cm catalyst charge of annular (5 mm×3 mm×2mm=external diameter×length×internal diameter) unsupported catalystaccording to example 1 of DE-A 10046957 (stoichiometry: [Bi₂W₂O₉×2WO₃]_(0.5) [Mo₁₂Co_(5.5)Fe_(2.94)Si_(1.59)K_(0.08)O_(x)]₁).

[0282] Configurations of a two-zone tube bundle reactor for the secondreaction stage which are advantageous in accordance with the inventioncan be designed as follows:

[0283] Everything as in the two-zone tube bundle reactor for the firstreaction stage. However, the thickness of the upper and lower catalysttube plates is frequently 100-200 mm, for example 110 mm, or 130 mm, or150 mm, or 170 mm, or 190 mm.

[0284] The aftercooler is dispensed with; instead, the lower openings ofthe catalyst tubes open into a hood which is connected to the vessel atthe lower end and has an outlet for the product gas mixture; the upperreaction zone is zone C and the lower reaction zone is zone D. Betweenthe outlet “aftercooler” and the inlet “reactor for the second reactionstage” is advantageously a means for feeding compressed air.

[0285] The catalyst tube and thermal tube charge (from top to bottom)can, for example, be as follows:

[0286] Section 1: length 20 cm steatite rings of geometry 7 mm×7 mm×4 mm(external diameter×length×internal diameter) as a preliminary bed.

[0287] Section 2: length 90 cm catalyst charge of a homogeneous mixtureof 30% by weight of steatite rings of geometry 7 mm×3 mm×4 mm (externaldiameter×length×internal diameter) and 70% by weight of coated catalystfrom section 4.

[0288] Section 3: length 50 cm catalyst charge of a homogeneous mixtureof 20% by weight of steatite rings of geometry 7 mm×3 mm×4 mm (externaldiameter×length×internal diameter) and 80% by weight of coated catalystfrom section 4.

[0289] Section 4: length 190 cm catalyst charge of annular (7 mm×3 mm×4mm=external diameter×length×internal diameter) coated catalyst accordingto preparative example 5 of DE-A 10046928 (stoichiometry:MO₁₂V₃W_(1.2)Cu_(2.4)O_(x)).

[0290] Alternatively, the first stage catalyst tube and thermal tubecharge can also have the following appearance (from top to bottom):

[0291] Section 1: length 50 cm steatite rings of geometry 7 mm×7 mm×4 mm(external diameter×length×internal diameter) as a preliminary bed.

[0292] Section 2: length 300 cm catalyst charge of annular (5 mm×3 mm×2mm=external diameter×length×internal diameter) unsupported catalystaccording to example 1 of DE-A 10046957 (stoichiometry:[Bi₂W₂O₉×2WO₃]_(0.5). [Mo₁₂Co_(5.6)Fe_(2.94)Si_(1.59)K_(0.08)O_(x)]₁).

[0293] The second stage catalyst tube and thermal tube charge can alsohave the following appearance (from top to bottom):

[0294] Section 1: length 20 cm steatite rings of geometry 7 mm×7 mm×4 mm(external diameter×length×internal diameter) as a preliminary bed.

[0295] Section 2: length 140 cm catalyst charge of a homogeneous mixtureof 25% by weight of steatite rings of geometry 7 mm×3 mm×4 mm (externaldiameter×length×internal diameter) and 75% by weight of coated catalystfrom section 3.

[0296] Section 3: length 190 cm catalyst charge of annular (7 mm×3 mm×4mm=external diameter×length×internal diameter) coated catalyst accordingto preparative example 5 of DE-A 10046928 (stoichiometry:MO₂V₃W_(1.2)Cu_(2.4)O_(x)).

[0297] In all of the first stage charges mentioned, the unsupportedcatalyst from example 1 of DE-A 10046957 can also be replaced by:

[0298] a) a catalyst according to example 1c of EP-A 15565 or a catalystto be prepared in accordance with this example, except having the activecomposition MO₁₂Ni_(6.5)Zn₂Fe₂Bi₁P_(0.0065)K_(0.06)O_(x).10 SiO₂;

[0299] b) example No. 3 of DE-A 19855913 as an unsupported hollowcylinder catalyst of the geometry 5 mm×3 mm×2 mm or 5 mm×2 mm×2 mm;

[0300] c) unsupported multimetal oxide II catalyst according to example1 of DE-A 19746210;

[0301] d) one of the coated catalysts 1, 2 and 3 of DE-A 10063162,except applied in the same coating thickness to support rings ofgeometry 5 mm×3 mm×1.5 mm or 7 mm×3 mm×1.5 mm.

[0302] In all of the abovementioned second stage charges, the coatedcatalyst can be replaced in accordance with preparative example 5 ofDE-A 10046928:

[0303] a) coated catalyst S1 or S7 from DE-A 4442346 having an activecomposition content of 27% by weight and a coating thickness of 230 μm;

[0304] b) a coated catalyst according to examples 1 to 5 of DE 19815281,except applied to support rings of geometry 7 mm×3 mm×4 mm having anactive composition content of 20% by weight;

[0305] c) coated catalyst having biphasic active composition ofstoichiometry (MO_(10.4)V₃W_(1.2)O_(x)) (CuMo_(0.5)W_(0.5)O₄)_(1.6),prepared according to DE-A 19736105 and having an active compositioncontent of 20% by weight, applied to the abovementioned 7 mm×3 mm×4 mmsupport.

[0306] According to the invention, the fixed catalyst bed 1 and thefixed catalyst bed 2 are advantageously otherwise selected in such a way(for example by dilution with, for example, inert material) that thetemperature difference between the heating point maximum of the reactiongas mixture in the individual reaction zones and the particulartemperature of the reaction zone generally does not exceed 80° C. Thistemperature difference is usually ≦70° C., frequently from 20 to 70° C.,and this temperature difference is preferably small. For safety reasons,the fixed catalyst beds 1 and 2 are also selected in a manner known perse to those skilled in the art (for example one dilution with, forexample, inert material) in such a way that the peak-to-salt-temperaturesensitivity as defined in EP-A 1106598 is ≦9° C., or ≦7° C., or ≦5° C.,or ≦3° C.

[0307] Aftercooler and reactor for the second stage are connected by aconnecting tube whose length is less than 25 m.

[0308] The reactor arrangement described and all other reactorarrangements and charges with fixed bed catalyst described in thisdocument can also be operated at high propene loadings, as described inthe documents WO 01/36364, DE-A 19927624, DE-A 19948248, DE-A 19948523,DE-A 19948241, DE-A 19910506, DE-A 10302715 and EP-A 1106598.

[0309] In this case, preference is given to the reaction zones A, B, C,D having the temperatures recommended in this document, except that thesecond reaction zone in each case, in accordance with the teaching ofthe abovementioned documents, has a higher temperature than the firstreaction zone in each case. The heating point temperature in the secondreaction zone in each case is always below that of the first reactionzone in each case.

[0310] However, at the propene loadings according to the invention, inthe procedure according to the invention results in an increasedselectivity of acrylic acid formation relative to the procedurerecommended for high propene loadings.

[0311] Generally, the volume-specific activity between two fixedcatalyst bed zones of a reaction stage can be experimentallydifferentiated in such a way that the same propene- oracrolein-containing starting reaction gas mixture is conducted underidentical boundary conditions (preferably the conditions of thecontemplated process) over fixed catalyst beds of the same length, buteach corresponding to the composition of the particular fixed catalystbed zone. The larger amount of propene or acrolein converted indicatesthe higher volume-specific activity.

[0312] In the examples and comparative examples which follow and also inthe reactor arrangement above, the annular shaped diluent bodies and theannular shaped catalyst bodies in the second reaction stage can also bereplaced by spherical shaped diluent bodies and spherical shapedcatalyst bodies (each having radius from 2 to 5 mm and having an activecomposition content of from 10 to 30% by weight, frequently from 10 to20% by weight). The statement relating to operation at high propeneloadings in the above documents retain their validity.

EXAMPLE AND COMPARATIVE EXAMPLES

[0313] a) Experimental Arrangement

[0314] First Reaction stage:

[0315] A reaction tube (V2A steel; external diameter 30 mm, wallthickness 2 mm, internal diameter 26 mm, length: 350 cm, and also athermal tube (external diameter 4 mm) centered in the middle of thereaction tube to receive a thermal element which can be used todetermine the temperature in the reaction tube over its entire length)is charged from top to bottom as follows:

[0316] Section 1: length 50 cm steatite rings of geometry 7 mm×7 mm×4 mm(external diameter×length×internal diameter) as a preliminary bed.

[0317] Section 2: length 140 cm catalyst charge of a homogeneous mixtureof 30% by weight of steatite rings of geometry 5 mm×3 mm×2 mm (externaldiameter×length×internal diameter) and 70% by weight of unsupportedcatalyst from section 3.

[0318] Section 3: length 160 cm catalyst charge of annular (5 mm×3 mm×2mm=external diameter ×length×internal diameter) unsupported catalystaccording to example 1 of DE-A 10046957 (stoichiometry: [Bi₂W₂O₉×2WO₃]_(0.5) [Mo₁₂Co_(5.5)Fe_(2.94)Si_(1.59)K_(0.08)O_(x)]₁).

[0319] The first 175 cm from top to bottom are thermostatted by means ofa salt bath A pumped in countercurrent. The second 175 cm arethermostatted by means of a salt bath B pumped in countercurrent.

[0320] Second Reaction Stage:

[0321] A reaction tube (V2A steel; external diameter 30 mm, wallthickness 2 mm, internal diameter 26 mm, length: 350 cm, and also athermal tube (external diameter 4 mm) centered in the middle of thereaction tube to receive a thermal element which can be used todetermine the temperature in the reaction tube over its entire length)is charged from top to bottom as follows:

[0322] Section 1: length 20 cm steatite rings of geometry 7 mm×7 mm×4 mm(external diameter×length×internal diameter) as a preliminary bed.

[0323] Section 2: length 90 cm catalyst charge of a homogeneous mixtureof 30% by weight of steatite rings of geometry 7 mm×3 mm×4 mm (externaldiameter×length×internal diameter) and 70% by weight of coated catalystfrom section 4.

[0324] Section 3: length 50 cm catalyst charge of a homogeneous mixtureof 20% by weight of steatite rings of geometry 7 mm×3 mm×4 mm (externaldiameter×length×internal diameter) and 80% by weight of coated catalystfrom section 4.

[0325] Section 4: length 190 cm catalyst charge of annular (7 mm×3 mm×4mm=external diameter×length×internal diameter) coated catalyst accordingto preparative example 5 of DE-A 10046928 (stoichiometry:Mo₁₂V₃W_(1.2)Cu_(2.4)O_(x)).

[0326] The first 175 cm from top to bottom are thermostatted by means ofa salt bath C pumped in countercurrent. The second 175 cm arethermostatted by means of a salt bath D pumped in countercurrent.

[0327] Gas Phase Oxidation:

[0328] The above-described first reaction stage is continuously chargedwith a starting reaction gas mixture 1 of the following composition, andthe loading and the thermostatting of the first reaction tube is varied:

[0329] from 6 to 6.5% by volume of propene,

[0330] from 3 to 3.5% by volume of H₂O,

[0331] from 0.3 to 0.5% by volume of CO,

[0332] from 0.8 to 1.2% by volume of CO₂,

[0333] from 0.01 to 0.04% by volume of acrolein,

[0334] from 10.4 to 10.7% by volume of O₂ and

[0335] the remainder ad 100% molecular nitrogen.

[0336] At the exit of the first reaction tube, a small sample iswithdrawn from the product gas mixture of the first reaction stage forgas chromatography analysis. Otherwise, the product gas mixture isconducted directly into the subsequent acrolein oxidation stage (toacrylic acid) by jetting in air at a temperature of 25° C. (reactionstage 2).

[0337] A small sample is likewise taken from the product gas mixture ofthe acrolein oxidation stage for gas chromatography analysis. Otherwise,the acrylic acid is removed in a manner known per se from the productgas mixture of the second reaction stage and a portion of the remainingresidual gas is reused for charging the propene oxidation stage (ascycle gas), which explains the acrolein content of the abovementionedcharging gas and the low variance of the feed composition.

[0338] In both reaction stages, the reaction gas mixture flows throughthe two catalyst tubes from top to bottom.

[0339] The pressure at the entrance to the first reaction stage variesbetween 2.3 and 3.1 bar as a function of the propene hourly spacevelocity selected. At the end of the reaction zones A, C there islikewise an analysis point. The pressure at the entrance to the secondreaction stage varies between 1.6 and 2.1 bar as a function of theacrolein hourly space velocity.

[0340] The results as a function of hourly space velocities and saltbath temperatures and also addition of air after the first reactionstage are shown by the following table (the letter E in brackets meansexample and the letter C in brackets means comparative example).

[0341] T_(A),T_(B),T_(c),T_(D) are the temperatures of the salt bathscirculated by pumping in the reaction zones A, B, C and D in ° C.

[0342] C_(PA) is the propene conversion at the end of reaction zone A inmol %.

[0343] C_(PB) is the propene conversion at the end of reaction zone B inmol %.

[0344] S_(WP) is the selectivity of acrolein formation and also ofacrylic acid by-production taken together after the first reaction stageand based on converted propene in mol %.

[0345] C_(AC) is the acrolein conversion at the end of reaction zone Cin mol %.

[0346] C_(AD) is the acrolein conversion at the end of reaction zone Din mol %

[0347] C_(PD) is the propene conversion at the end of reaction zone D inmol % (based on the starting amount of propene in reaction zone A).

[0348] S^(AA) is the selectivity of acrylic acid formation after thesecond reaction stage and based on converted propene in mol %.

[0349] R is the molar ratio of molecular oxygen:acrolein in the startingreaction gas mixture 2.

[0350] M is the amount of air added after the first reaction stage in l(STP)/h. T^(maxA), T^(maxB), T^(maxC), T^(maxD) are each the highesttemperature of the reaction gas mixture within the reaction zones A, B,C and D in ° C.

[0351] b) Results TABLE Propene hourly space velocity (I(STP)/I · h)T_(A) T_(B) T^(maxA) T^(maxB) C_(PA) C_(PB) S_(WP) M R 130 (E) 319 319384 351 66.7 95.1 97.7 404 1.40 130 (E) 327 313 400 330 70.3 94.8 98.3404 1.38 130 (C) 311 325 361 372 60.8 95.0 97.1 404 1.41 185 (C) 322 336380 368 64.5 94.9 98.0 574 1.39 185 (C) 310 344 353 383 57.2 95.2 97.3574 1.40 Acrolein hourly space velocity (I(STP)/I · h) T_(C) T_(D)T^(maxC) T^(maxD) C_(AC) C_(AD) C_(PD) S^(AA) 160 (E) 260 260 302 27680.7 99.3 95.1 95.4 108 (E) 262 259 312 275 84.8 99.3 94.8 95.8 104 (C)257 262 285 291 64.0 99.3 95.0 94.9 152 (C) 263 269 303 287 78.8 99.394.9 95.8 147 (C) 257 278 278 310 61.3 99.3 95.2 95.0

1. A process for partially oxidizing propene to acrylic acid in the gasphase under heterogeneous catalysis by conducting a starting reactiongas mixture 1 which comprises propene, molecular oxygen and at least oneinert gas, and contains the molecular oxygen and the propene in a molarO₂:C₃H₆ ratio of ≧1 in a first reaction stage over a fixed catalyst bed1 which is arranged in two spatially successive reaction zones A, B, thetemperature of reaction zone A being a temperature in the range from 290to 380° C. and the temperature of reaction zone B likewise being atemperature in the range from 290 to 380° C, and whose activecomposition is at least one multimetal oxide comprising the elements Mo,Fe and Bi, in such a way that reaction zone A extends to a propeneconversion of from 40 to 80 mol % and the propene conversion on singlepass through the fixed catalyst bed 1 is ≧90 mol % and the accompanyingselectivity of acrolein formation and also of acrylic acid by-productiontaken together is ≧90 mol %, the temperature of the product gas mixtureleaving the first reaction stage is optionally reduced by cooling andmolecular oxygen and/or inert gas are optionally added to the productgas mixture, and then the product gas mixture, as a starting reactiongas mixture 2 comprising acrolein, molecular oxygen and at least oneinert gas and containing the molecular oxygen and the acrolein in amolar O₂:C₃H₄O ratio of ≧0.5, is conducted in a second reaction stageover a fixed catalyst bed 2 which is arranged in two spatiallysuccessive reaction zones C, D, the temperature of reaction zone C beinga temperature in the range from 230 to 320° C. and the temperature ofreaction zone D likewise being a temperature in the range from 230 to320° C., and whose active composition is at least one multimetal oxidecomprising the elements Mo and V, in such a way that reaction zone Cextends to an acrolein conversion of from 45 to 85 mol % and theacrolein conversion on single pass through the fixed catalyst bed 2 is≧90 mol % and the selectivity of acrylic acid formation assessed overall reaction zones, based on converted propene, is ≧80 mol %, thesequence in time in which the reaction gas mixture flows through thereaction zones corresponding to the alphabetic sequence of the reactionzones, wherein a) the hourly space velocity of the propene contained inthe starting reaction gas mixture 1 on the fixed catalyst bed 1 is <160l (STP) of propene/l of fixed catalyst bed 1·h and ≧90 l (STP) ofpropene/l of fixed catalyst bed 1·h; b) the volume-specific activity ofthe fixed catalyst bed 1 is either constant or increases at least oncein the flow direction of the reaction gas mixture over the fixed bedcatalyst bed 1; c) the difference T^(maxA)−T^(maxB), formed from thehighest temperature T^(maxA) which the reaction gas mixture has withinreaction zone A and the highest temperature T^(maxB) which the reactiongas mixture has within reaction zone B is ≧0° C.; d) the hourly spacevelocity of the acrolein contained in the starting reaction gas mixture2 on the fixed catalyst bed 2 is <145 l (STP) of acrolein/l of fixedcatalyst bed 2·h and ≧70 l (STP) of acrolein/l of fixed catalyst bed2·h; e) the volume-specific activity of the fixed catalyst bed 2increases at least once in the flow direction of the reaction gasmixture over the fixed bed catalyst bed 2; and f) the differenceT^(maxC)−T^(maxD), formed from the highest temperature T^(maxC) whichthe reaction gas mixture has within reaction zone C and the highesttemperature T^(maxD) which the reaction gas mixture has within reactionzone D, is ≧20° C.
 2. A process as claimed in claim 1, wherein thedifference T^(maxA)−T^(maxB) is ≧3° C. and ≦70° C.
 3. A process asclaimed in claim 1, wherein the difference T^(maxA)−T^(maxB) is ≧20° C.and ≦60° C.
 4. A process as claimed in claim 1, wherein the differenceT^(maxC)−T^(maxD) is ≧3° C. and ≦60° C.
 5. A process as claimed in claim1, wherein the difference T^(maxC)−T^(maxD) is ≧3° C. and ≦40° C.
 6. Aprocess as claimed in claim 1, wherein the hourly space velocity of thepropene contained in the starting gas mixture on the fixed catalyst bed1 is ≧100 l (STP) or propene/l·h and ≦150 l (STP) or propene/l·h.
 7. Aprocess as claimed in claim 1, wherein the hourly space velocity of thepropene contained in the starting gas mixture on the fixed catalyst bed1 is ≧110 l (STP) or propene/l·h and ≦145 l (STP) or propene/l·h.
 8. Aprocess as claimed in claim 1, wherein the difference T_(B)−T_(A)between the temperature of reaction zone B, T_(B), and the temperatureof reaction zone A, T_(A), is ≧−10° C. and ≦0° C.
 9. A process asclaimed in claim 1, wherein the difference T_(C)−T_(D) between thetemperature of reaction zone D, T_(D), and the temperature of reactionzone C, T_(C), is ≧−10° C. and ≦0° C.
 10. A process as claimed in claim1, wherein the temperature of reaction zone A is from 305 to 365° C. 11.A process as claimed in claim 1, wherein the temperature of reactionzone A is from 310 to 340° C.
 12. A process as claimed in claim 1,wherein the temperature of reaction zone C is from 250 to 300° C.
 13. Aprocess as claimed in claim 1, wherein the temperature of reaction zoneC is from 260 to 280° C.
 14. A process as claimed in claim 1, whereinthe active composition of the fixed catalyst bed 1 is at least onemultimetal oxide active composition of the general formula IMo₁₂Bi_(a)Fe_(b)X¹ _(c)X² _(d)X³ _(e)X⁴ _(f)O_(n)  (I) where X¹=nickeland/or cobalt, X²=thallium, an alkali metal and/or an alkaline earthmetal, X³=zinc, phosphorus, arsenic, boron, antimony, tin, cerium, leadand/or tungsten, X⁴=silicon, aluminum, titanium and/or zirconium, a=from0.5 to 5, b=from 0.01 to 5, c=from 0 to 10, d=from 0 to 2, e=from 0 to8, f=from 0 to 10 and n=a number which is determined by the valency andfrequency of the elements other than oxygen in I.
 15. A process asclaimed in claim 1, wherein the active composition of the fixed catalystbed 2 is at least one multimetal oxide active composition of the generalformula IV Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶_(g)O_(n)  (IV) where the variables are defined as follows: X¹=W, Nb,Ta, Cr and/or Ce, X²=Cu, Ni, Co, Fe, Mn and/or Zn, X³=Sb and/or Bi,X⁴=one or more alkali metals, X⁵=one or more alkaline earth metals, X⁶Si, Al, Ti and/or Zr, a=from 1 to 6, b=from 0.2 to 4, c=from 0.5 to 18,d=from 0 to 40, e=from 0 to 2, f=from 0 to 4, g=from 0 to 40 and n=anumber which is determined by the valency and frequency of the elementsother than oxygen in IV.